Account.java

public class Account {

    private String owner;
    private int amount;

    public String getOwner() {
        return owner;
    }

    public int getAmount() {
        return amount;
    }

    public Account(String owner) {
        this.owner = owner;
    }

    public Account(String owner, int amount) {
        this(owner);
        amountAuthorizedCreation(amount);
        this.amount = amount;
    }

    public static void amountAuthorizedCreation(int amount) {
        if (amount < 0) {
            throw new RuntimeException("Amount has to be greater than or equal to 0.-");
        }
    }

    public static void amountAuthorizedOperations(int amount) {
        if (amount <= -2000) {
            throw new RuntimeException("Amount has to be greater than or equal to -2000.-");
        }
    }

    public void withdraw(int amountToRetrieve) {
        amountAuthorizedOperations(this.amount - amountToRetrieve);
        this.amount -= amountToRetrieve;
    }

    public void deposit(int addAmount) {
        amountAuthorizedOperations(this.amount + addAmount);
        this.amount += addAmount;
    }

    public void transfer(int amount, Account dst) {
        dst.deposit(amount);
        withdraw(amount);
    }

    @Override
    public boolean equals(Object obj) {
        return (obj instanceof Account) && (((Account) obj).getOwner() == this.owner)
                && (((Account) obj).getAmount() == this.amount);
    }

    public static void main(String[] args) {
        Account p1 = new Account("Person 1", 100);
        Account p2 = new Account("Person 2", 100);

        p1.transfer(2101, p2);

        System.out.println(p1.getAmount());
    }
}